Februaby 15, 1S95.] 



SCIENCE. 



V 



called for. Of two regions, similar in all 

 respects except that one is made of resist- 

 ant rocks, and the other of weak rocks, the 

 fii*st will develop a stronger relief dnring its 

 mature dissection than the second. The 

 Great Plains of the "West are often referred 

 to as a region of considerable elevation, in 

 which, however, the rivers are unable to cut 

 deep vallej's on account of the rapid disin- 

 tegration of the tributary slopes, and the 

 consequent necessity of maintaining steep- 

 sloping channels in order that the streams 

 may do theii- work of bearing the plentiful 

 waste of the land to the sea. 



All this series of considerations is con- 

 fused if it is said that a river which has es- 

 tablished an equality between its capacity 

 and its task is 'at baselevel. ' From whatever 

 profile of slope it began to work on, it has 

 developed a profile of equilibrium, as certain 

 French writers would phrase it ; or, follow- 

 ing a suggestion by G. K. Gilbert (Chicago 

 Jouraal of Geology, II., 189-1, 77), it has 

 graded its slope ; it is a graded river ; it is 

 almost balanced between degi'ading and ag- 

 grading its valley, and most of its activity 

 may be given to lateral sapping. No better 

 English term than ' grade ' has been sug- 

 gested for the expression of this important 

 idea. 



GEOMOEPHOGENY OF NORTHERN CALIFORNIA. 



The second contribution to the general 

 subject alluded to above is bj^ A. C. Lawson, 

 in account of the Geomorphogeny of the 

 coast of northern California (Bull. Dept. 

 Geol., Univ. of Cala., I., 1894, 241-242), 

 which students of this new-named subject 

 will do well to consult. Although only the 

 report of a rapid reconnoissance, the paper 

 announces the determination of a well- 

 marked, uplifted and dissected peneplain, 

 in which a fully developed system of subse- 

 quent drainage is exhibited on an extensive 

 scale. The district is recommended to 

 students as an inviting field for further in- 



vestigation. The author brings out the 

 point that a constructional mass of resistant 

 rocks will never at any stage of its denuda- 

 tion yield a topography that may be reach(Kl 

 at certain stages in the denudation of a mass 

 of weaker rocks ; and he therefore suggests 

 that in the accounts of topographic devel- 

 opment, or geomorphogeny, a factor should 

 be introduced indicative of the rate as well 

 as of the stage of degradation of the region 

 concerned. 



THE ESSENTIAL PRINCIPLES OF BASE- 

 LEVELLING. 



The results gained in the two papers 

 mentioned above, and in many other similar 

 articles, are based on the essential princi- 

 ples of baseleveUing : Any region must in 

 time be reduced to a nearly featureless 

 peneplain close to sea level ; during the pro- 

 gress of its denudation, the forms assumed 

 follow a tolerably well defined sequence, de- 

 pending chiefly on the structure of the 

 wasting mass ; the features and arrange- 

 ment of the drainage lines are essentially 

 systematic and not arbitrary in their de- 

 velopment. A generally accepted corollary 

 of these princiijles is that a surface of de- 

 nudation, having faint relief and no control 

 by structure, can be produced onlj- close to 

 its controlling baselevel ; and that such a 

 surface represents the peneplain stage, at- 

 tained close to the end of the cj'cle of denu- 

 dation in which it was developed. It is 

 evident that if a plain of denudation can be 

 produced at a considerable altitude above 

 baselevel, and independent of structure, 

 then the conclusions of various investiga- 

 tors regarding land movements, based on 

 the occurrence of elevated, warped or 

 faulted peneplains, must be critically re- 

 vised. It therefore behooves those who ac- 

 cept and employ the doctrine of baselevel- 

 ling to examine carefully any alternative 

 hypothesis by which peneplains are ex- 

 plained independently of baselevels. 



